The purpose of this comparative study was to measure how much blood must be wasted before accurate coagulation results can be obtained from heparinized double lumen Hickman catheters. A random sample of 20 was employed. Results: After discards of 5, 10, 15, 20, and 25 mL of blood, samples were collected for PT, APTT, and fibrinogen (FIB) concentrations. Peripheral blood was simultaneously drawn for the same tests. Hickman results were compared with those derived from the peripheral blood samples and were expressed as % of PTs within l second, PTTs within 3 sec and FIB within 75mg/dL of the peripheral samples. PTs are "accurate" after a 15 mL discard. FIB's are "accurate" after a 5mL discard. Although the APTT is not "accurate" after a 25 mL discard, 95% of samples were in the normal range (22.5 - 34.8).